R and I APASS magnitudes in Seqplot

I apologize in advance for asking a question that may have been already answered in the past.

I see that the Seqplot software can generate BVRI sequences extracted from the APASS catalog. As far as I know, the APASS catalog provides B and V Johnson magnitudes and g, r and i Sloan magnitudes. Therefore, I would like to know how the R and I APASS magnitudes in Seqplot are obtained from the original APASS database.

I can't comment at the moment as to which sloan filter conversion formula's Arne has been using for Rc & Ic but I can point you to a good website that offers the majority of the sloan filter conversion formulas in use with various surveys:

Looking at the aforementioned SDSS transformation web page, it does not appear that Jester provides a way to recover Rc or Ic individually, only the Rc-Ic index. There is a V-R formula, but since the 'c' designation appears elsewhere with R and I, I presume this V-R is Johnson.

For seqcat/seqplot, we use the Jester et al. (2005) transformations for Rc&Ic, in conjunction with the already-available V. Seqcat actually contains V and the astrophysical color indices (B-V), (V-Rc), (Rc-Ic) and (V-Ic). Only when comp stars are selected does software convert the magnitude&colors into separate magnitudes by appropriate additions and subtractions. Note that the color part of the Jester transformations is small, on the order of 0.09*index for (V-Rc) and essentially zero for (Rc-Ic), showing that the Sloan r'/i' filters are pretty close in bandpass to the Cousins equivalents.

Tom, Jester's equations are for Cousins Rc and Ic. No professional that I know of uses Johnson Rj/Ij.

The SDSS transformation web page is inconsistent in its use of notation. Jester's set of equations are listed as applying to UBVRcIc, and an equation is given for Rc-Ic. No equation is given for V-Rc, it is specified as V-R. Sloppy!

Anyway, I have another question. If you go to the web page for u'g'r'i'z' -> ugriz conversion, you find the following:

The redward cuts are imposed because the u'g'r'i'z' standard star system is not well-determined for stars redder than about M0. "

If you grind through the math, r'-i' <= 0.60 translates to Rc-Ic < 0.82 (approximately). This is a more severe constraint than the Rc-Ic <=1.15 Jester specifies. It also seems a little hokey for Jester to put the color constraint on the output of the transformation rather than the input. What is going on here?